~~
Table I.
~
Differentiating Titration Results of Various Phenol Mixtures
Mixture 2,4Dinitrophenol, m-nitrophenol, phenol 2,4-Dinitrophenol, p-nitrophenol, phenol 2,4,6-Trinitrophenol, 2,4-dinitrophenol, o-nitrophenol 2,4,6-Trinitrophenolj 2,4-dinitrophenol, o-nitrophenol, phenol
M1. Hydroxide Theory 1.90 1.92 1.96 1.90 1.93 1.96
...
1.58 1.63
...
1.58 1.63 1.63
alcohol is more accurate than in pyridinr. For example, the titration of 4.00 nil. of a 0.1013X o-ethylphenol (dissolved i n benzene) in tsrt-butyl alcohol solvent required 3.43 ml. of 0.11YJI titrant, (standardized against benzoic acid and corrected for carbonate impurities). The same titration in pyridine solvent required 3.52 ml. When the titration time was lengthened b y 5 minutes, 3.58 ml. were required. Vapor phase chro-
Titrant Required 1.88 1.90 1.96 1.89 1.91 1.95 1.56
Deviation from Theory, hI1. -0.02
1.58
0.00 -0.01
1.152
-0.01 0.00
1.62 1.22 1.58 1.63
-0.02
0.00 -0.01 -0.02 -0.01
...
0.00
matograms of o-ethylphenol did not show the presence of impurities. The purity of o-ethylphenol calculated from the results of the titration in butyl alcohol was 99.70/,. The titration results in pyridine, however, gave purities of 102 and 1047,, depending upon the time taken for titration. Evidently, the decomposition of tetrabutylammonium hydroxide in pyridine leads to a n appreciable error in the
determination of a weakly acidic phenol. For an acid stronger than phenol, decomposition does not take place until an excess of base is added. Since the inflection point of the titration curve is thus virtually unaffected, an accurate measurement of the acid content can be obtained. LITERATURE CITED
(1) Bruss, D. B., Harlow, G. ,I,,ANAL. CHEW30, 1835 (1958). ( 2 ) Cundiff, R. H., Rlarkunas, P. C., Ibid., 28, 792 (1956). (3) Fritz. J. S.,Yamaniura. S. S..Ibid.. 29, 1079 (1957). (4) Hummelstedt. L. E. I..’ Huine. D. N.. ‘ Ibid., 32, 1792 (1960). (5) Malmstadt, H. V., Vassallo, D. 8., Zbid., 31, 206 (1959). (6) Marple, L. W.,Fritz, J. S.,Zbid., 34, 796 (1962). (7) Pritchett, J. IT., M. S. thesis, Iowa State Universitv. 1960. (8) van der Heijde, H. B., .inai. Chim. Acta 16, 392 (1957). (9) Tanianiura, S. S., Ph.D. thesis, Iowa State College, 1957. RECEIVED for revieTv January 15, 1962. Accepted April 19, 1962. Division of Analytical Chemistry, 141st Meeting, ACS, Washington, D. C., March 1962. Contribution No. 1089. Work performed in the hmes Laboratory of the U. S. Atomic Energy Commission.
Potentiometric Titration of 2,4-Dinitrophenylhydrazones of Ketones with Sodium Nitrite JOSEPH
G. BALDINUS and IRVIN ROTHBERG 8, French laboratories, Philadelphia I , Pa.
Smith Kline
-
b 2,4 Dinitrophenylhydrazones (DNPH’s) of many ketones can b e titrated with sodium nitrite. The Sample is dissolved in sulfuric acid, tetrahydrofuran and hydrochloric acid are added, and the end point is determined potentiometrically. DNPH’s with C=O, +CH2, or a single hydrogen adjacent to C=N generally cannot be determined. Various other structural features limit the applicability of the method, and these are discussed. DNPH’s of aldehydes are not titratable-with two exceptions. Results are presented for 33 DNPH’s; for these and similar DNPH’s the method should prove useful.
T
of phenylhydrazine with sodium nitrite has been reported (19). This paper describes a procedure in which this titrant is used t o determine 2,4-dinitrophenylhydrazones (DNPH’s). The titrations are carried out in a tetrahydrofuran medium containing sulfuric and hydrochloric HE TITRATION
924
0
ANALYTICAL CHEMISTRY
acids. DNPH’s of aldehydes cannot be determined, although two exceptions b-ere encountered. Also, many DNPH’s of ketones were not titratable, which further limits the applicability of the method. When it can be used, the method is simple, an excellent potentiometric break is obtained, and the results are reproducible.
Materials. Carbonyl compounds used t o prepare D Y P H ’ s in Tables I and I1 were: trifluoroacetone from Columbia Organic Chemicals Co., Inc. ; benzylacetone from Aldrich Chemical Co.; p-hydroxyacetophenone, p-methoxyacetophenone, and 2pentanone, technical grade chemicals from Eastman Kodak Co.; cyclohexylacetone and m-methoxyacetophenone, synthesized materials t h a t were homogeneous by gas chromatography and had elemental analyses in agreement with theory; the remaining materials were ’IVhite Label, Eastman Kodak Co. chemicals. Mesityl oxide D N P H was prepared from Eastman Kodak, White Label, diacetone alcohol (9). For Table 111, Matheson Cole-
man & Bell and Eastman Kodak Chemicals were used, except ethyl pyruvate which mas obtained from Brothers Chemical Go. The hydrazones were prepared by the diglyme procedure (1O), and mere crystallized from pyridine or pyridine-water to a constant melting point. Apparatus and Reagents. A Photovolt Model 110 or a Beckman Model G p H meter, equipped with platinum and calomel electrodes. Sodium nitrite, 0.05N aqueous solution. Standardized against sulfanilamide, U.S.P. reference standard, using aqueous hydrochloric acid as the solvent. Keep concentration of acid between 10 t o 20% (TV./W.): outside these limits the break gets progressively smaller. Procedure. Weigh approximately 1 meq. of D S P H into a beaker equipped with a magnetic stirrer. .4dd 10 ml. of concentrated sulfuric acid, a n d dissolve the sample. S h u t off the stirrer, and from a pipet add approximately 100 ml. of tetrahydrofuran. R u n t h e solvent in carefully, so t h a t t h e tetrahydrofuran forms a layer on t o p of t h e sulfuric acid. Now t u r n the stirrer gradually, as heat is generated
when the two solvents are mixed. The tetrahydrofuran may start to boil, but this is easily controlled. S e x t add 10 ml. of concentrated hydrochloric acid, cool, and titrate potentiometrically with sodium nit'rite. Add the titrant rapidly in the beginning, but slowly near the end point, in increment,s of 0.1 ml. The potential drifts during the titration, so record the potent,ial exactly 1 minute aft'er each addition of titrant. Calculate the end point by the second derivative niet'hod (8). Experimental. T h e solubility of D S P H ' s in a number of solvent mixt'ures was investigated. Mixtures used were sulfuric acid in combination with acetic acid, acetonitrile, methyl cellosolve, a n d tetrahydrofuran. Of these solvent pairs, only tetrahydrofuran and sulfuric acid mas completely satisfact'ory. This mixture not only dissolved the DNPH's but kept them in solution throughout the titration. T o get a potentiometric response, it was necessary to add concentrated hydrochloric acid. I n general, 10 ml. was adequate, although the DXPH's of trifluoroacetone and silicylaldehyde gave poor breaks and required additional acid. Except. for these two compounds, little improvement was noted when more acid was added. Hydrobromic acid was tried as a catalyst, but gave breaks inferior to those obtained with hydrochloric acid. The length of time a D K P H remained in sulfuric acid had little effect. Samples of propiophenone D X P H were held in sulfuric acid for 4 hours, yet recoveries were quantitative. RESULTS
Results obtained b y titration of various DNPH's are given in Tables I and 11. 411 these DNPH's reacted with one equivalent of nitrous acid. DNPH's that could not be determined are listed in Table 111. DISCUSSION
A few generalizations can be drawn from Table 111. Compounds in which C=K is flanked b y both a single hydrogen (C-H), and an alkyl group consume more nitrite than theory but do not give a n end point. Examples are the DNPH's of 1,l -diphenylacetone and methyl isopropyl ketone (Tables I and 11). JF7hen C-H is more remote from C = S as in methyl isobutyl ketone, D K P H (Table 11) titration is normal. A benzene ring in conjugation with C=N nullifies the effect of C-H, so that isobutyrophenone D S P H (Table I) becomes titratable. Perhaps, even in this compound, C-H is participating in some secondary reaction as the titration result (97.77,) is lower than would be expected. If a methylene group adjacent to C = S is qCH, or is part of a ring, i t behaves like C-H. DKPH's of dibenzyl ketone, methyl benzyl ketone,
Table I.
Determination of DNPH's of Aromatic Aldehydes and Ketones b y Titration with Sodium Nitrite
2,4-Dinitrophenylhydrazone of Acetophenone p-Fluoroacetophenone p-Chloroacetophenone p-Bromoacetophenone p-Methylacetophenone p-Methoxyacetophenone m-Methoxyacetophenone p-Hydroxyacetophenone p-Phenylacetophenone m-Sitroacet,ophenone 3,4-Dimethylacetophenone 2,4-Dihydroxyacetophenone
Propiophenone Butyrophenone Isobutyrophenone Valerophenone 1-Acetonaphthone 2-Acetonaphthone Salicylaldehydei Benzy lacetone Vanillin a e e
lI.P., O
c.
247-248 243-245 237-239 234-235 260-262
227-228 189-190 259-261 239-241 230-232 255-257 246-251 192-194 188-190 160-162 167-168 255-257 258-260drk 250-251 127-129 268d3k
XP., Lit." 250 252' 235-23f1~ 237 258 227-22V
18ge 261 2411 228 2558 242-243
191 190 163 166 25gi 26Zdsk 252 131-132h 271d,k
5; Purit) 99,4b 99.4,99.5 99,2,99.5 99.8,100.0 99.7,100.2 99.1,99.7 99.0,99.9 98.5)98.6 100.1,100.3 98.9,100.1 98.7,100.3 98.2,98.3 99.5,99.7 98.5,98.7 97.1,97.2,97.5,98.0 99.4,99.6 93.3,98.4 99.6,99.6 101.9,102.1 99.0,99.5,100, I , 100.2 99.4,100.0
(4)
Average of 7 determinations with std. dev. f 0.33%. (1) (7) (3) (11) (5) (6) (IS) 25 nil. of HC1 used in titration.
; f
j
Decomposes. Determination of DNPH's of Aliphatic Ketones Nitrite 2,4-Dinitrophenylhydraxone M.P., M.P., of c. Lit." 125-126 126 Acetone 143-145 143-144 2-Pentanone 2-Butanone 112 116-117 3-Pentanone 154-157 156 Ethyl acetoacetate 90-91 93 Ethyl levulinate 100-102 102 204-205 206.5b Levulinic acid 125-126 125 Pinacolone 115-117 ... Cyclohexyl acetone 95-96 95 Methyl isobutyl ketone Trifluoroacetonec 137-139 139 Mesityl oxide 199-20 1 203
Table 11,
a
b y Titration with Sodium
yo Purity 99.2,98.6 100.0,100.9 99.8,99.9 99.2,99.4 100.1, 100.8 98.6,98.7 98.0,98.1, 98.4 98.4,99.2 100.4,100.5 99.2,99.5 100.8, 101.1,101.4 98.0,98.0,99.4,99.6
(4) (6)
25 ml. of HCl used in titration.
desoxybenzoin, cyclohexanone, and ahydrindone fall into this category. Again, when the offending group is farther away from C=S, the D S P H titrates satisfactorily (benzyl and cyclohexyl acetone, Tables I and 11). Sote, however, that in desoxybenzoin, a benzene ring in conjugation with C=N has not abolished the interference from QCH,. I n analogy to isobutyrophenone, one might have expected this D K P H to be titratable. A terminal methylene group interferes even though it is distant from C=A-; thus, 5-hexene2-one D N P H reacts with nitrite, but does not give an end point. X double bond in conjugation n-ith C=K is present in trio DNPH's. Mesityl oxide (Table 11) titrates satisfactorily; benzalacetone, however, gives
erratic results (100 to 105%). K h e n C=O is conjugated with C=N, the D N P H turns sluggish and cannot be titrated. DKPH's such as 2,3-pentanedione (mono), ethyl pyruvate, and pyruvic acid (Table 111) are examples. Ortho substituents can hinder the reaction with nitrite. Compare, for example, 3,4- and 2,4-dimethylacetophenone. Whereas, the 3,4-DSPH titrates smoothly, the Z4-compound reacts sluggishly, and titration is not feasible. The ortho effect is not general, however, since the D N P H of 2,4dihydroxyacetophenone (Table I) can be titrated. The derivatives of benzoin, acetylacetone, and benzophenone are wholly indifferent to the titrant. Benzoin DXPH turns black and probably does VOL. 34, NO. 8. JULY 1962
925
Table 111. DNPH’s That Cannot Be Determined by Titration with Sodium Nitrite
2,4-Dinitrophenylhydrazone of 1,l-Diphenylacetone Methyl isopropyl ketone Dibenzyl ketone
Benzalactone 5-Hexen-2-one
2,3-Pentanedione (Mono) Methyl benzyl ketone Ethyl pyruvate Desoxybenzoin Pyruvic acid Cyclohexanone 2,4-Dimethylacetophenone a-Hydrindone 2,5-Dichloroacetophenone Formaldehyde Acetylacetone“ Benzaldehyde Glyoxal (bis) Isobutyraldehyde 2,3-Pentanedione (bis) Hexaldehyde Benzophenone Heptaldehyde Benzoin Not a true DXPH.
not survive in the strong acid solution, while acetylacetone forms a pyrazole rather than a hydrazone n5th 2,4dinitrophenylhydrazine ( 2 ) . The in-
ertness of benzophenone DNPH is attributable to the two benzene rings in conjugation with C=N. Because of the C-H effect, one might anticipate that aldehyde DNPH’s TI ould consume nitrite, but would not give end points. The DNPH’s of hexaldehyde, heptaldehyde, isobutyraldehyde, and hydrocinnamaldehyde behave as expected. Salicylaldehyde and vanillin DSPH’s are exceptions and are titratable. Although vanillin gives an excellent break, only a small inflection is obtained with salicylaldehyde which may account for the poor titration results (102%). I n benzaldehyde and formaldehyde DSPH’s, the reactivity is so diminished that titration is not possible. All D S P H ’ s tested were soluble in the titration medium except the bis derivatives of glyoxal and 2,3-pentanedione. Summarizing, many DKPH’s can be titrated with nitrite. Certain methylene and C-H groups interfere, n-hich severely limits the applicahility of the method.
LITERATURE CITED
(1) Bergniann, E. D., Berkovic, S., Ikan, R., J . Am. Chem. SOC.78, 6037 (1956). (2) Brady, 0. L., J . Chem. SOC.1931,
.-”.
766
(3) Buchta, E., Keidinger, H., -4nn. Chem. 580, 109 (1953). (4) Cheronis, X. D., Entrikin, J. B., “S,emimicro Qualitative Organic hnalp i s , ” 2nd ed., Interscience, Ken. York, 1957. ( 5 ) Grignard, V. (ed.), “Trait6 de chimie Organique,” Vol. 7, p. 1004, Masson et Cie, Paris, 1950. (6) Heilbron, ,I,., “Dictionary of Organic Compounds, Oxford University Press, New York, 1953. ( 7 ) Jones, L. il.,Hancock, C. X., J . Org. Chem. 25, 226 (1960). (8) Lingane, J. J., “Electroanalytical Chemistry,” 2nd ed., p. 93, Interscience, Ken, York, 1958. (9) Shine, H. J., J . Ow. Chem. 24, 1790 (1959). (10) Ibzd., 24, 252 (1959). (11) Szmant, H. H., Planinsek, H. J., J . .4m. Chem. SOC.72, 4042 (1950). (12) Vulterin, J., Z$ka, H., Chem. Listy. 50, 364 (1956). (13) Williams, J. W.,Osborn, J. V., J . Am. Chem. SOC.61, 3438 (1939). RECEIVEDfor review August 18, 1961. Resubmitted May 1, 1962. Accepted May 1, 1962.
Voltammetry and Amperometric Titrimetry of Cyanide at the Rotating Platinum Microelectrode FUJIKO SHINOZUKA and JOHN T. STOCK Department of Chemisfry, University o f Connecticuf, Storrs, Conn.
b
Voltammetric studies a t the rotating platinum microelectrode have confirmed the magnitude of the formation constant
K.4
of the ion Ag
from
argentocyanide (4). Sources of error in the amperometric titration with silver nitrate of low concentrations of cyanide in 0.1 M sodium hydroxide have been examined. Provided that volatilization of hydrogen cyanide i s prevented, titration in 0.1M sodium sulfite a t a potential of -0.20 volt vs. S.C.E. i s more satisfactory. Chloride or bromide in concentrations up to 100 times that of cyanide does not interfere significantly. Interference b y iodide or b y traces of sulfide may b e eliminated b y titration at -0.85 volt.
T
HE amperometric titration with silver nitrate of concentrations of from 8 X to 0 , l M of cyanide in 0.1V sodium hydroxide has been described by Laitinen, Jennings, and Parks (6). By means of a mercury-mercuric iodide-potassium iodide reference electrode, the rotating platinum electrode
926
ANALYTICAL CHEMISTRY
was maintained a t -0.23 volt n ith respect to the saturated calomel electrode. The current remained small up to the region of the argentocj-anide equivalence point; further addition of titrant caused a progressive rise in current. It was shown by Kolthoff and Stock (4) that silver, hydroxyl. and argentocyanide ions react to form hydroxyargentocyanide ion, Ag (OH)( C S )-; electroreduction of this coniplex ion is presumably the cmse of the post equivalence point current. I n our hands the careful titration by this method of cyanide in concentrations of about 10-3N yielded results that were persistently low by sereral per cent; results were better when the titration TI as performed rapidly. The factors governing the amperometric titration of lov concentrations of cyanide have, therefore, been critically examined and two new procedures have been 11-orked out. EXPERIMENTAL
Reagents, Apparatus, and Technique. T h e approximately 0 . 2 X po-
tassium cyanide stock solution n as
standardized immediately before use ( 2 ) . Measurements m-ere made a t room temperature (23’ to 26’ C.). The apparatus was generally as described previously (3, 4),but most electrical measurements were made with a Leeds &- Korthrup Type E Electrochemograph that was operated manually. Unless the solution in the cell contained sulfite, it was deoxygenated as completely as possible by bubbling nitrogen through it. -411 potentials were measured with respect to the saturated calomel electrode (S.C.E.). The sensitivity of the platinum ITire microelectrode, TT hich was rotated a t 600 r.p.m. by a Sargent synchronous rotator, was about 105 pa. per millimole of silver per liter in 0 , l M potassium nitrate. Before each run, the electrode was soaked in hot 10M nitric acid, short-circuited in 0.1M perchloric acid against the S.C.E. until the current became very small, then thoroughly rinsed with distilled water ( 5 ) . I n amperometric studies, the titrant was delivered from a Gilmont microburet. Magnetic stirring was used only during titrant addition. Recommended Titration Procedures, Transfer t h e cyanide solu-
tion t o a 50-ml. beaker t h a t contains a magnetic stirrer bar. Immerse t h e
